The present invention is directed to a multilayer system for protecting components exposed to severe environmental and thermal conditions such as the hostile environment present in gas turbine engines.
A major limitation in the efficiency and emission of current gas turbines is the temperature capability (strength and durability) of metallic structural components (blades, nozzles and combustor liners) in the engine hot section. Although ceramic thermal barrier coatings are used to insulate metallic components, thereby allowing the use of higher gas temperatures, the metallic component remains a weak link. Such components must allow for the possibility of coating loss from spallation or erosion.
Silicon-containing ceramics are ideal materials for high temperature structural applications such as heat exchangers, advanced gas turbine engines, and advanced internal combustion engines. They have excellent oxidation resistance in clean oxidizing environments due to the formation of a slow-growing silica (SiO2) scale. Reduced durability in high temperature environments containing molten salts, water vapor or a reducing atmosphere can limit their effectiveness. Molten salts react with silica scale to form liquid silicates. Oxygen readily diffuses through liquid silicates and rapidly oxidizes the substrate. High water vapor levels lead to hydrated silica species (Si(OH)x) and subsequent evaporation of protective scale. Complex combustion atmospheres containing oxidizing and reducing gases form SiO2 and then reduce it to SiO(g). In situations with low partial pressure of oxidant, direct formation of SiO(g) occurs. All of these reactions can potentially limit the formation of a protective silica scale and lead to accelerated or catastrophic degradation.
Examples of silicon-containing ceramics are SiC fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs), SiC fiber-reinforced Si3N4 matrix composites (SiC/Si3N4 CMCs), carbon fiber reinforced SiC ceramic matrix composites (C/SiC CMCs), and monolithic silicon carbide or silicon nitride. A primary problem Si-containing ceramics face is rapid recession in combustion environments due to the volatilization of silica scale via reaction with water vapor, a major product of combustion. Therefore, use of silicon-containing ceramic components in the hot section of advanced gas turbine engines requires development of a reliable method to protect the ceramic from environmental attack. One approach in overcoming these potential environmental limitations is to apply a barrier coating which is environmentally stable in molten salts, water vapor and/or reducing atmosphere.
An early environmental barrier coating system (EBC) consisted of two layers, a mullite (3Al2O3.2SiO2) coat and a yttria-stabilized zirconia (YSZ) top coat. The mullite coat provided bonding, while the YSZ top coat provided protection from water vapor. Mullite has a good coefficient of thermal expansion match and chemical compatibility with Si-based ceramics. However, the relatively high silica activity of mullite and the resulting selective volatilization of silica cause its rapid recession in water vapor. This EBC provided protection from water vapor for a few hundred hours at 1300xc2x0 C. During longer exposures, however, water vapor penetrated through cracks in the mullite and attacked the Si-containing substrate, leading to coating delamination.
Another EBC with improved performance was developed as part of a NASA High Speed Research-Enabling Propulsion Materials (HSR-EPM) Program in joint research by NASA, GE, and Pratt and Whitney. The EBC consisted of three layers: a silicon bond coat, an intermediate coat consisting of mullite or mullite and barium strontium aluminosilicate (BSAS), and a BSAS top coat. The mullite, mullite and BSAS, and BSAS layers were applied by a modified plasma spray process developed at the NASA Glenn Research Center as disclosed in U.S. Pat. No. 5,391,404, which is incorporated by reference herein in its entirety. The EBC was applied to SiC/SiC CMC combustor liners used in three Solar Turbine Centaur 50s gas turbine engines. The combined operation of the three engines resulted in the accumulation of tens of thousands of hours without failure at a maximum combustor liner temperature of about 1250xc2x0 C. A drawback of this BSAS-top coat EBC is that when applied to the solar turbine SiC/SiC liners it suffered from substantial BSAS recession after engine testing.
Protective layers for Si-containing ceramic substrates, and for other substrate materials which may also be subject to degradation under the harsh hot section engine component type of environment, including silicon/metal alloys and oxide ceramics, are needed which improve upon the performance of current systems by having a longer life or capability to withstand higher operating temperatures without failure.
The present invention is directed to a multilayer article which includes a substrate comprising a compound selected from the group consisting of a ceramic compound, a Si-containing metal alloy and combinations thereof. In particular, the ceramic compound may be a Si-containing ceramic or an oxide ceramic with or without Si. One preferred oxide ceramic is a mullite (3Al2O32SiO2)-containing ceramic. The multilayer article also includes an outer layer and at least one intermediate layer located between the outer layer and the substrate. The outer layer comprises one of the following compounds having a low coefficient of thermal expansion: rare earth (RE) silicates; at lease one of a) hafnia (HfO2) and b) hafnia (HfO2)-containing composite oxides; and zirconia (ZrO2)-containing composite oxides. Components of gas turbine engines, internal combustion engines and heat exchangers may be comprised of the multilayer article of the present invention.
Although the outer layer may be referred to as a top coat, it need not be a coating per se. Also, other layers may be placed on top of the outer layer (i.e., further from the substrate than the outer layer). It should be understood that terms such as upper, lower, top, bottom and the like are used in this disclosure for purposes of illustration and should not be used to limit the invention, since these relative terms depend upon the orientation of the substrate. The intermediate layer is typically a layer that is applied to the substrate or to a layer(s) on the article. However, some substrate materials, such as mullite-containing materials, inherently form an intermediate layer (e.g., mullite). Whether applied or inherent, both constitute intermediate layers as these terms are used in this disclosure. Also, the term composite oxide used herein means a compound or a mixture containing at least two oxides.
In one embodiment of the invention the outer layer comprises a low CTE rare earth silicate having a formula selected from the group consisting of (1) RE2O3.SiO2, (2) 2RE2O3.3SiO2, (3) RE2O3.2SiO2 and combinations thereof. RE is a rare earth element selected from the group consisting of Sc, Dy, Ho, Er, Tm, Yb, Lu, Eu, Gd, Tb and combinations thereof. Exemplary rare earth oxides are Sc2SiO5, Er2SiO5, Yb2SiO5 and combinations thereof. One preferred rare earth silicate has the formula RE2O3.SiO2, characterized by a low CTE X2 phase, where RE is Sc, Dy, Ho, Er, Tm, Yb, Lu and combinations thereof Another preferred rare earth silicate has the formula RE2O3.SiO2, characterized by a low CTE X1 phase, where RE is Eu, Gd, Tb and combinations thereof. The rare earth silicate-containing outer layer is not used on a C/C ceramic substrate (carbon fiber-reinforced carbon matrix composite). A preferable substrate on which the rare earth silicate outer layer is used comprises a Si-containing ceramic compound.
In a second embodiment of the invention the outer layer comprises at least one of a) hafnia and b) a hafnia-containing composite oxide comprising hafnia and a compound selected from the group consisting of mullite, barium strontium aluminosilicate, calcium aluminosilicate, magnesium aluminosilicate, rare earth silicate, alumina, tantalum oxide, niobium oxide, silica, titania and combinations thereof. In particular, the hafnia-containing composite oxide comprises a compound selected from the group consisting of HfO2.SiO2, HfO2.TiO2, xHfO2.Ta2O5 (where 5xe2x89xa6xxe2x89xa67, x being preferably 6), and combinations thereof In another aspect of the invention, the outer layer comprises a mixture of HfO2 and an oxide selected from the group consisting of mullite, barium strontium aluminosilicate, calcium aluminosilicate, magnesium aluminosilicate, rare earth silicate, Al2O3, Ta2O5, Nb2O5 and combinations thereof.
In a third embodiment of the invention the outer layer comprises a zirconia-containing composite oxide comprising zirconia and a compound selected from the group consisting of silica, titania, tantalum oxide and combinations thereof In particular, the zirconia-containing composite oxide comprises a compound selected from the group consisting of ZrO2.SiO2, ZrO2.TiO2, xZrO2.Ta2O5, where 5xe2x89xa6xxe2x89xa67, x being preferably 6, and combinations thereof. This outer layer is preferably used on a substrate which is susceptible to recession resulting from volatilization upon exposure to water vapor. A preferred substrate on which the zirconia-composite oxide outer layer is used comprises a Si-containing ceramic compound.
The intermediate layer can comprise a single layer or multiple layers and is defined herein as being located between the outer layer and a bond layer on the substrate or, if no bond layer is used, between the outer layer and the substrate. A mullite-containing intermediate layer comprises a compound selected from one of (1) mullite (3Al2O3.2SiO2) and (2) mullite and a compound selected from the group consisting of barium strontium aluminosilicate or BSAS (x BaO.(1xe2x88x92x) SrO.Al2O3.2SiO2) where 0xe2x89xa6xxe2x89xa61, calcium aluminosilicate or CAS (CaO.Al2O3.2SiO2), magnesium aluminosilicate or MAS (2MgO.2Al2O3.5SiO2) and combinations thereof. An especially suitable intermediate layer used in combination with any of the outer layers comprises mullite and BSAS.
The intermediate layer may include a chemical barrier layer comprising a compound selected from the group consisting of mullite, hafnia (HfO2), hafnia silicate (e.g., HfSiO4), rare earth silicate (e.g., RE2SiO5 where RE is Sc or Yb), and combinations thereof. The chemical barrier layer is preferably located between an intermediate layer and the outer layer, more particularly, is in contact with the outer layer and even more particularly is between and contiguous with the outer layer and the mullite-containing layer. Another mullite chemical barrier layer may be disposed adjacent to the bond layer or substrate.
A silicon-containing bond coat comprised, for example, of silicon (Si) or silicides, such as molybdenum-silicon alloys and niobium-silicon alloys, can be disposed in contact with the substrate.
The substrate comprises one of the following compounds: a Si-containing ceramic, such as silicon carbide (SiC), silicon nitride (Si3N4), composites having a SiC or Si3N4 matrix, silicon oxynitride, and silicon aluminum oxynitride; a Si-containing metal alloy, such as molybdenum-silicon alloys (e.g., MoSi2) and niobium-silicon alloys (e.g., NbSi2); and an oxide ceramic such as mullite-containing ceramics (e.g., a mullite matrix with ceramic fibers, such as alumina fibers, dispersed in the matrix). The substrate may comprise a matrix reinforced with ceramic fibers, whiskers, platelets, and chopped or continuous fibers.
Other features, details and advantages of the invention will be apparent from the attached drawings and detailed description that follows.